Process for chromizing



Dec. 7, 1965 SAMUEL ETAL 3,222,212

PROCESS FOR CHROMIZING Filed Nov. 29, 1962 iZg'z/f INVEN'TORS r a A. 0%Y me I4 662 izing without accompanying disadvantages.

United States Patent O 3,222,212 PROCESS FOR CHROMIZING George A.Samuel, Pas de Calais, France, and Jerome V. Bell, Wilmington, Del.,assignors to Alloy Surfaces Company, Inc., Wilmington, Del., acorporation of Delaware Filed Nov. 29, 1962, Ser. No. 240,858 13 Claims.(Cl. 117--107.2)

The present application is a continuation-in-part of our copendingapplication Serial No. 172,231, filed February 9, 1962, now abandonedfor Process for Chromizing and Product, which is a continuation-in-partof our copending application Serial No. 160,764, filed December 20,1961, now abandoned for Process for Chromizing and Product, which is inturn a continuation-in-part of our copending application Serial No.119,085, filed June 23, 1961, now abandoned for Metal Difiusion. Subjectmatter relating to the Chromizing Product has been divided and appearsin divisional application Serial No. 307,263, filed September 6, 1963,for Process of Chromizing and Porduct.

The present invention relates to the chromizing of ferrous metal parts,particularly low carbon steel and ingot iron, although permissiblyincluding medium and high carbon steel.

-A purpose of the invention is to circulate chromizing gases in auni-directional manner in contact with the surface of ferrous metalwork, which may suitably take the form of an open coil, but may be ofsome other char- -acter, and thus to obtain more rapid and moreeconomical chromizing with lower expendiutres of chromium and withgreater throwing power.

A further purpose is to chromize more effectively at low 1 temperature.

the work suitably being an open coil, so that a stream of chromizinggases which has passed through a source of chromium not in contact withthe work is directed under positive pressure so that it flows in contactwith the work and then returns to the source of chromium withoutdivision or short-circuiting.

A further purpose is to circulate a carrier gas containing ahalogen-containing gas through a source of chromium not in contact withthe workin a chromizing retort, and to maintain the concentration ofhalogen in the halogen-containing gas at a suffiicently low level as toavoid priming the source of chromium or depositing a large excess ofliquid or solid chromium halide on the source of chromium which wouldtend to react with moisture and interfere with chromizing in a lateroperation.

A further purpose, particularly when chromizing with hydrogen andhydrogen bromide (or bromine or chromous bromide), is to control theconcentration of hydrogen bromide or the like so as to obtain moreeffective chrom- This is particularly important where the source ofchromium is porous and may become impregnated with liquid or solidchromium bromide which will be difiicult to remove. the preferredprocess of the invention, the partial pressure of hydrogen bromide orthe like is kept at a value which will come close to saturating thevapor phase with chromium bromide, while at the same time not being highenough to impregnate the source of chromium with liquid or solid3,222,212 Patented Dec. 7, 1965 Ice chromous bromide and not high enoughto deposit a great excess of liquid or solid chromous bromide, whichwhen the retort is opened will react with oxygen in the air and moistureand cause difficulty on the next cycle of operation.

A further purpose is to utilize a combination of carrier gas andhalogen-containing gas which has an inlet dew point of below F., andpreferably below -ll0 F., and is substantially free from oxygen.

A further purpose is to control the quantity of oxygen (as such or asoxides which can react) present in the retort so that the dew point ofthe exit gases will be below -40 F. or preferably be below 60 F., andwill in any case not be higher than +5 F.

A further purpose is to protect the work during heating up to atemperature not exceeding 1100 F. by a non-explosive gas such asnitrogen and then to eliminate the presence of nitrogen and at highertemperatures avoid nitrogen and thereby avoid nitriding effects.

A further purpose is to chromize using ferrochrome or chromium or othersource of chromium having a particle size range between 60 mesh and Athe source of chromium being permissibly either porous chromium or densechromium.

A further purpose is to agitate the particles forming the source ofchromium during chormizing preferably by fiuirizing the bed so that theparticles are supported by the chromizing gas stream and thus surroundedby the chromizing gas.

A further purpose is to employ a surface area of the source of chromiumto the area of the work which will be between 0.2 to l and 10 to 1 orhigher, preferably between 0.8 to 1 and 10 to 1, and most desirablybetween 3to1and5to l.

A further purpose is to conduct the chromizing preferably at lowtemperatures in the range from 1600 to 1800 F., particularly when usingthe system involving a source of chromium, hydrogen as a carrier gas,and hydrogen bromide (or bromine or chromous bromide) as ahalogen-containing gas. Suitable times will be of the order of 5 to 10hours or longer.

A further purpose is to use a coil space between adjoining laps of theorder of 0.135 to 0.22 inch.

A further purpose is to employ the principles of the invention on steelsor other ferrous metal products of low carbon content, having carboncontents below 0.03% and preferably below 0.003%

A further purpose is to apply the principles of the invention to steelswhich have been stabilized for example by titanium, columbium, vanadiumor tantalum, so that the carbon content uncombined with the alloyingelement is kept very low, below 0.03% and preferably 0.003%.

A further purpose is to render the steel more suitable for chromizing bydecarburizing the steel, preferably in the same operation, as byintroducing wet hydrogen, and in a temperature of the range of 1300" to1400" F. and then thoroughly drying the retort before chromizing.

A further purpose is to obtain a chromized steel which will benon-aging, having an aging index of from 0 to 2% A further purpose is toredistribute any superficial skin which is abnormally high in chormiumand which may tend to be brittle by diffusion of the chromium into thebase metal by continuing the exposure to hydrogen at or about chromizingtemperature while removing the halogen so that chromizingwill cease.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate one of the numerous typesof apparatus which may be used in carrying out the process of theinvention.

FIGURE 1 is a vertical cross sectional view of one form of apparatus forsubjecting open coils of strip or sheet material to chromizingtreatment.

FIGURE 2 is a horizontal cross sectional view taken substantially on theline 2-2 of FIGURE 1, and illustrating the form and arrangement of thetrays for carrying the source of chromium.

FIGURE 3 is an enlarged vertical cross sectional view takensubstantially on line 33 of FIGURE 2, of several of the upper trayscarrying the source of chromium.

The particular furnace shown in respect to its constructional detailsforms no part of the present invention.

Describing in illustration but not in limitation and referring to thedrawings:

Extensive use has been made in the prior art of chromizing processes inwhich a powder pack including a source of chromium such as ferrochromeand inert bodying material are placed in contact with the work and acompound such as ammonium chloride, ammonium bromide, ammoniumbifiuoride and ammonium iodide is caused to break down liberatinghalogen-containing gas to promote chromizing.

Efforts have also been made in the past to chromize by employing ahalogen-containing gas such as hydrogen chloride, and a carrier gas suchas hydrogen which were passed over a source of chromium to causeformation of chromous chloride and then to pass the chromous chlorideover the work and out of the retort. Both of these processes have beensubject to the limitation that they are relatively slow, cumbersome andexpensive. They also are not always reliable, particularly because ofinfiltration of oxygen and in some cases because of the presence ofmoisture.

In the case where it is desired to produce chromized products from sheetor strip, there are two alternate possibilities. The practice usuallyused has been to produce the final components and then to chromize themafter forming. This is subject, however, to the disadvantage that thereis a very poor work load in the retort and the parts often suffer fromheat distortion. Also, the effects of previous cold work are destroyed.

On the other hand, it has not been possible in the prior art to obtaineffective chromizinng of large areas in coil form.

In the case of the pack method, a serious limitation has been the verypoor heat transfer. Another limitation in the pack method has been thepoor throwing power which made it important to position the work inintimate contact with the pack. Poor finish on the work has also been adifficulty, as well as poor physical properties of the steel sincetreatment temperatures have been relatively high and also the treatmentshave been rather prolonged.

Where chromizing gas has been generated outside the retort and passedthrough the retort and then out of the retort, in the prior art, thewaste in chemicals has been very serious. Furthermore, the directionalcharacter of the gas flow has produced shadowing effects in thedeposition which limit the throwing power transverse to the direction ofgas flow.

The previous systems have also been very uneconomical from thestandpoint of chemical utilization. The present invention is concernedparticularly with overcoming the difficulties noted and particularlyobtaining a more economical, more efficient and more effectivechromizing operation.

In accordance with the invention, the chromizing gases, which willpreferably consist of a carrier gas plus a halogen-containing gas, aresequentially and repetitively passed through or over a source ofchromium such as metallic chromium or ferrochrorne, and then in contactwith the work, and then back through or over the source of chromium toregenerate and again in contact with the work. This is accomplishedunder forced circulation as from a pump (or fan), sothat theentiregaseous content of the retort can be recycled in a time as short as onesecond.

This process, therefore, permits very efficient charging of the retort,since it is no longer necessary to place in contact with, or interposebetween the many layers of ferrous metal work, a powder pack as in thewell known powder pack prior processes. The throwing power of the retortgaseous content is so great that it is possible to effectively chromizerelatively tremendous areas of work as for example coiled sheet or stripin which the individual laps of the coils are opened sufficiently toallow gas circulation through the coil. The best prior art practice bythe pack method in chromizing sheets has utilized a spacing of threesheets to the inch. By the technique of the present invention it isreadily possible to chromize with laps of coil which are a s close as 4to 5 to the inch.

Since there is no longer any need for refractory materials such asalumina, the time of heating up the retort can be reduced to a minimum.Furthermore, the efficiency of charging the retort is greatly increasedbecause no extra space must be occupied by the powder pack.

As compared with the prior process in which chromizing gases have beengenerated outside the retort, the present process uses the same gascontent over and over, instead of discharging it from the retort when ithas once come in contact with the work. As a consequence, it is onlynecessary to introduce or withdraw gas from the retort once thechromizing operation has started in sufficient quantity to maintain asuperatmospheric pressure in the retort or to compensate forinfiltration of air and to otherwise remove objectionable material suchas water vapor which may have been present in the retort or may havebeen formed due to reduction of oxides present. have been formed due toreduction of oxides present. Therefore, the chemical consumption isreduced to a minimum.

By reusing the gas, the waste of chromium and other metal halides, whichwould result from wasting the gas when it has once been used forchromizing is avoided, and also the halogen itself is conserved.

Use has been made in the prior art of static gaseous processes, in whichthe gaseous content which is to accomplish chromizing is essentially allcontained within a sealed retort, in some cases with venting provided.These processes are extremely slow, and as compared with them, thethrowing power of the present process is infinite since the gases can bemade to follow any desired course within the retort which will bringthem in contact with all parts of the work.

Efforts have been made in the past to chromize using a rotating retortcontaining gases and a powdered or granulated pack. In this case,however, the gas at any point is a haphazard mixture of regenerated andunregenerated gas unlike the gaseous content of the retort 1n thepresent invention.

The present invention lends itself partlcularly to the chromizing ofvery large charges, for example of open coil steel sheet, since there isno appreciable limitation of soaking time and no serious problem of heattransfer. Single coils as large as 20,000 pounds may be chromized in asingle retort operation and in a time of the order of only 10 hours atchromizing temperature.

From the standpoint of the product, there have been serious difficultiesin chromizing by prior art practices. When using the pack method, undercertain circumstances it is likely that chromium from the source ofchromium will sinter on the surface and produce roughness. There hadalso been difficulty through producing relatively abnormally lightchromizing at certain places where gaseous diffusion or where packmaterial were not present.

The present invention lends itself particularly to the production ofvery high quality chromided steel products since chromizing is carriedout in the absence of nitrogen and the chromizing operation can relievefrom the presence of dissolved nitrogen. The absence of nitrogen is alsoI advantageous because it prevents nitriding the source of chromiumwhich deteriorates the source of chromium and also tends to contaminatethe steel.

The invention is particularly suitable for chromizing at lowtemperatures, which tends to minimize or avoid harm to the steel, eitherby grain growth or due to distortion. I

The chromized case which is obtained in the present invention is of verysuperior quality from the standpoint both of ductility and corrosionresistance because of the extremely low carbon content of the ferrousmetal work, resulting in low chromium carbide content in the chromizedlayer. The 'carbon content of the case may be as low as 0.02% carbonwhen the case is formed on 20 gage steel which has been decarburizedprior to chromizing.

Many prior art processes which have used a pack have depended uponpriming of the pack so that it is impregnated with solid or liquid phasechromous chloride. It has been found that the priming of the source ofchromium is likely to cause serious difliculties in subsequentprocessing. Certain processes require priming as a separate technique,which is time consuming and expensive, and use the priming material asthe source of chromizing potential until it is so depleted that it mustbe replaced. For example, in some instances in the prior art extremecare must be exercised in protecting the primed source of chromium frommoisture and oxidation, as otherwise this will lead to very high dewpoints in the chromizing gases, impairing the quality of the chromizing.

GASEOUS CONTENT While the process of the present invention can operateeffectively using any one of the halogens, as for example hydrogenchloride, chromous chloride, hydrogen fluoride, chromous fluoride,chlorine, hydrogen bromide, bromine, chromous bromide, hydrogen iodide,chromous iodide and various other metal halides having sufficiently highvapor pressure at chromizing temperature, such as aluminum chloride andaluminum bromide, there are great advantages in the process of thepresent invention in using hydrogen bromide, chromous bromide, orbromine, as later explained, and particularly when operating attemperatures of 1600" to 1800 -F. These halogens are referred to hereinas halogen acid gases.

There is also an important advantage in using hydrogen as a carrier gaswhen a bromine chromizing system is being employed, that is, hydrogenplus hydrogen bromide, or hydrogen plus bromine, or hydrogen pluschromous bromide. Hydrogen is readily available, can be obtained withvery low oxygen contents and nitrogen contents and is not objectionablefrom the standpoint of effect on the steel. Hydrogen tends to promotethe reduction type of chromizing reaction as follows:

Thus, the hydrogen carrier gas acts as a conveyor of thehalogen-containing gas, and tends to reduce oxygen which may be presentin the retort for example as oxide. Hydrogen procured With an oxygencontent less than 2 ppm. and a dew point less than 1 l0 -F., and anitrogen content less than 2 ppm. is obtainable at relatively low cost,and 'can be used in copious quantities to flush out the system and beadded during chromizing in such quantities as are required to remove anyoxygen introduced 'by leakage and also to remove water formed either byreaction with oxides which are present, or water driven off from theinterior of the restort which was formerly absorbed or chemicallycombined. It will be understood that in many cases a porous source ofchromium will be used which may give off moisture, oxygen or otherobjectionable materials under chromizing conditions. The source ofchromium likely will contain some chromium oxide, and the hydrogencarrier gas will reduce this to metallic chromium when 6 supplied insufficient quantities, and thus maintain low dew points in the exitgases as later explained.

It will of course by evident that adequate inspection techniques shouldbe instituted to insure the purity of the hydrogen, as this is acritical feature of the process.

A convenient way to determine whether there is sufiicient freedom frommoisture vapor and oxide in the retort is to determine the dew point ofexit gases. Dew points as high as +5 F. can be tolerated in the exitgases, although for best results, the dew points in the exit gasesshould be lower than 40 or preferably lower than F.

In a retort having an internal free space of approximately 300cubiefeet, it was desirable to maintain a flow of hydrogen carrier gasof the order of 2000 cubic feet per hour during the drying period priorto mixing with the halogen-containing. gas, and after chromizing startedand halogencontaining gas was added, it was found that a flow rate of300 cubic feet per hour was adequate (input of hydrogen at standardconditions). These gases are measured at F. and one atmosphere.

In order to do effective chromizing with hydrogen bromide or achremically equivalent amount of chromous bromide or bromine in acarrier gas of hydrogen, it is quite important that a minimumconcentration lof hydrogen bromide by volume of 2.00% be maintained inthe temperature range from 1600 to 1800 F. where bromide chromizing ismost effective. If the concentration of hydrogen bromide or equivalentis less than 2.00% by volume, satisfactory chromizing is not obtained.

In many cases, it is also important that a maximum percentage ofhydrogen bromide or a chemically equivalent percentage of chromousbromide or bromine be maintained in the mixture with hydrogen carriergas which is sufiiciently low to prevent the deposition in the retort ofsolid or liquid chromous bromide in any substantial quantity. If solidor liquid chromous bromide deposits, difliculty is then likely to beencountered by absorption of oxygen and moisture when the retort isopened, and a great deal of delay and waste of hydrogen will occur instarting up the next heat before adequately low dew points can beobtained in the retort atmosphere and effective chromizing can start.

While the presence of deposited solid or liquid chromous bromide on thesurface of the retort or on the surface of massive (dense) chromium orfer-rochrome is objectionable, for the reasons stated and also becauseof the tendency to clog up gas passages, traps, and the like, thedifiiculty is much more serious when porous ferrochrome is used, forexample ferrochrome produced by powder metallurgy techniques includingvacuum sintering. In this case the solid, or more particularly liquidchromous bromide, tends to impregnate the porous ferrochnome and isparticuraly diflicult to free from oxygen and moisture in subsequentcycles, acting as a continual drag on the chromizing potential of theretort.

It will, of course, be evident that if minor amounts of solid or liquidchromous bromide are deposited on the surface of the retort, they can beremoved by washing or dissolving in water, but this procedure is noteffective with porous ferrochrome.

In the preferred procedure, sufficient chromous bromide is formed tojust saturate the gas phase inside the retort. From this standpoint theconcentration of hydrogen bromide or the equivalent should be as closeas convenient to but not substantially exceeding the upper limits set.

The following table shows maximum and minimum limits for hydrogenbromide in percentages by volume measured at 70 F. and one atmospherefor various temperatures in the range from 1600 to 1800 F. This appliesonly to mixtures of hydrogen bromide in hydrogen, although chemicallyequivalent percentages will likewise apply for mixtures of chromousbromide with hydrogen and of bromine with hydrogen.

It will be evident that the quantities of gases can be determined byweighing the gas cylinders from which the gases are supplied.

Where bromine is used instead of hydrogen bromide, with the hydrogencarrier gas, it is desirable to bubble the hydrogen through the bromineor pass the hydrogen through a chamber in which the bromine is beingevaporated. However, this procedure is not recommended in case theprocess of the invention is used with chlorine be cause of the extremehazard of an explosion when chlorine is introduced into hydrogen.

It will be evident that mixtures of halogen-containing gases whichcontain different halides may be employed in the process of theinvention if desired, such as hydrogen bromide plus hydrogen chloride;hydrogen bromide plus hydrogen iodide; hydrogen iodide plus hydrogenchloride; hydrogen fluoride plus hydrogen bromide; hydrogen fluorideplus hydrogen chloride; hydrogen fluoride plus hydrogen iodide, etc.

SEQUENTIAL OPERATION It is of great importance in some aspects of theprocess of the present invention to provide a positive means ofcirculating the gases in the retort, along with passages or chamberswhich will secure a sequenital flow of substantially all the gases inthe retort.

The positive circulation of the gases will conveniently be applied byany suitable pumping means, such as a fan or blower. Experience hasindicated that for best results the pump should be capable ofrecirculating substantially all the gases of the retort at a high rate,conveniently once each second. In order to achieve this result, it hasbeen found that in a retort having a free volume of the order of 300cubic feet, a circulation flow of 13,600 cubic feet per minute (measuredat 70 F. and one atmosphere pressure) is satisfactory.

It is quite important that the direction of gas flow will cause thegaseous atmosphere to pass through the source of chromium, so as toregenerate the gases and form new chromous. bromide or other chromoushalide, and then to pass the gases after regeneration over or around thework so as to accomplish chromizing. The gases should then be returnedas promptly as possible to the intake of the pump or fan in order tocome in contact again with the source of chromium. It is important forthe success of the reaction that both the source of chromium and alsothe work be held at chromizing tempera ture and that the gases be atchromizing temperature, so that if the regeneration is accomplished bypassing the gases through the source of chromium in an adjoining orseparate retort chamber, such chamber should be adequately maintained ata temperature of approximately the chromizing temperature. It will beevident that once a chromizing cycle is completed, the source ofchromium will be depleted in chromium content at the surface. It is verydesirable prior to the next cycle or by the beginning of the neXt cycleto promote surface chromium restoration by diffusion from the interiorof the individual particle to build up the chromium concentration at thesurface of the source of chromium.

This is accomplished in the present invention by allowing a period ofseveral hours to elapse in the next chromizing cycle during which thesource of chromium is held at an elevated temperature, preferably equalto or greater than the chromizing temperature, prior to the introductionof halogen and the inception of chromizing.

In the preferred apparatus, the source of chromium is located adjacentthe heating means, and is therefore, at a slightly higher temperaturethan the work. This promotes more rapid chromium restoration in theinitial stages of the new cycle prior to the introduction of halogen,and also allows the chromium source to supply chromium to thehalogen-containing gases more rapidly during the chromizing cycle. Allconnecting passages must also be maintained at chromizing temperature toavoid condensation of the chromium halides.

The pump, of course, can be anywhere in the system.

SOURCE OF CHROMIUM The source of chromium for use in the process of theinvention can either be chromium or an alloy high in chromium such asferrochrome. Where ferrochrome is used, it should be the low carbon andlow nitrogen grade so as not to deposit nitrogen or carbon, and shouldpreferably have a chromium content in excess of a typical analysis byweight being as follows:

Chromium maximum. Silicon 2% maximum. Carbon 0.015% maximum. Nitrogen0.025% maximum. Iron Substantially balance.

The ferrochrome can be of the powder metallurgy porous type or of thedense solid type if desired. Chromium can be used. A typical analysisis:

Chromium 98.5% minimum. Carbon 0.015% maximum. Nitrogen 0.025% maximum.

It has been found that higher superficial chromium contents in the casemay be obtained when chromium metal is used as source of chromium ratherthan ferrochrome. Thus, in a typical example, superficial chromiumcontent in the case of 44% was obtained using chromium metal Whereaswhen undepleted ferrochrome was used, the superficial chromium contentof the case was 42%. In both instances a hydrogen bromide-hydrogensystem was used at a temeprature of 1650 F., and the chromizing wascarried on at chromizing temperature.

While various sizes of chromium or ferrochrome particles can be used, inmost cases it will be desirable to employ particles larger than 60 mesh(Tyler standard mesh per linear inch) and not in excess of inch. Acommon nominal size is inch or inch. The size is chosen in order tostrike a reasonable balance between the surface area of the source ofchromium which should desirably be large and the ease with which thegases can penetrate the source of chromium.

Ferrochrome particles having the following sizes were studied in detailand actual counts of the particles per pound were made:

The particles Were generally cubical or rectangular. Accordingly, it waspossible to calculate the surface area in square feet per pound offerrochrome particles and the following data were obtained:

Table III Surface area in Size in inches square feet per pound Thesurface area of the source of chromium should bear a relationship to thesurface area of the work, which will be in the range of 0.2 to 1 to 10to 1, and preferably 0.8 to 1 to 10 to 1, and most desirably between 3to 1 and 5 to 1. For best results the ratio should be about 4 to 1.Larger proportions of surface area of the source of chromium may beemployed, but are not necessary in common practice. Smaller area ratiosdecrease the chromium content of the case.

The area ratios have an effect on the superficial chromium percentageand the average chromium percentage in the case. Using treatments at1650 F. for hours at heat, with between 3.2 and 4.2% by volume ofhydrogen bromide in hydrogen and a steel of AISI 1010 composition whichwas decarburized before chromizing to a carbon content of 0.002%, thevalues shown in the following table were obtained:

While chromizing using the broad principles of the present invention canbe carried out at temperatures in the range between 1500 and 2300 F.,the preferred temperature range should be between 1600 and 1800 F. andpreferably between 1600 and 1750" F. where the hydrogen bromide-hydrogensystem is employed. Chromous bromide, the effective chromizing agent,has an adequate partial vapor pressure within this temperature range.

Table V VAPOR PRESSURE OF CHROMOU'S BROMIDE IN ATMOSPHERES TemperatureF.: Atmospheres 1600 0.041 1800 0.19

It will be evident that if a lower rate of transportation of gases isemployed, it may be desirable to use a system having higher vaporpressures such as the chromous iodide system in order to get aseffective chromizing under the new conditions.

The preferred temperature range in using the hydrogen bromide-hydrogensystem is 1675 to 1700 P. where open coils are being treated in therange of gages from 24 through 16 for thickness of the sheet. Using thistemperature and a time not exceeding 10 hours chromized case depths of0.0015 inch minimum can readily be obtained at 1700 F., and after hoursat 1660 F. case depths of 0.0011 inch can readily be obtained.

For low carbon steels having an uncombined carbon content not exceeding0.03% where the effective chromizing agent is chromous bromide, it ispreferred ordinarily not to go to temperatures above 1750 F. Where 10the carbon content is greater than 0.08% temperatures no higher than1900 F. are preferred, but in such cases chromous chloride or chromousfluoride will preferably be used as the effective chromizing agent.

STEEL COMPOSITION The most important aspect of the composition of thesteel or other ferrous metal is its carbon content. It is important inobtaining ductile cases on open coil strip and sheet to have carboncontents not exceeding 0.03% and preferably not exceeding 0.003%.

Steels and ingot irons of such low carbon contents are availablecommercially.

In many cases, however, it is preferable to obtain the low carboncontent by starting with an initial moderately low carbon content suchas AISI 1010 steel, and then further reducing the carbon content bydecarburizing. The decarburizing can be carried out as a separateoperation using any well recognized decarburizing technique. It is,however, preferable to decarburize prior to the chromizing cycle but inthe same heat. This can be accomplishcd rather readily by subjecting theopen coil or other ferrous metal work to wet hydrogen at a temperaturein the range of 1300 to 1400" F. An appreciable quantity of moisture ispresent in the hydrogen. An important aspect of the present invention isthat the source of chromium is not primed and therefore it is notnecessary to eliminate the effect of this moisture on primed chromium orferrochrome prior to chromizing. After decarburization is complete,reducing the carbon content to a level below 0.003% and suitably of theorder of 0.002%, it is merely necessary to pass dry hydrogen through thesystem during the heat up period to chromizing temperature in order toobtain adequately low dew points on the exit gases in order to startchromizing.

It will be understood that where decarburization is carried out in thepresence of ferrochrome or chrome using wet hydrogen at say 1300 to 1400F., the source of chromium itself is oxidized and to some extentcarburized. However, after the steel has given up substantially all ofits carbon in the form of effluent carbon monoxide gas and even afterthe carbon monoxide concentration of the efiiuent gases has reached alow value of 0.01% by volume, the use of wet hydrogen is continued forseveral hours in order to reduce the carbon content of the chromiumsource to its original value. Subsequent introduction of dry hydrogenincident to the elevation of temperature for chromizing, results indeoxidizing of the source of chromium, so that effective chromizing canbe carried out at chromizing temperature.

It sometimes is desirable at the beginning of chromizing to introduce aricher halogen content than will subsequently be used. This acceleratesthe formation of the proper amount of chromous halide especially whenvery low flow rates of carrier gas are employed. Thus it may beadvantageous in this case to use an initial content of halogen in thehydrogen stream of say ten to twelve percent of hydrogen bromide, andafter a short time, say one hour, reduce the halogen content to thelimits previously referred to.

In some cases the steel may be stabilized by the alloying of titanium,columbium, vanadium or tantalum in a quantity of the order of at leastfour times the carbon content. Such steels should not have an uncombinedcarbon content in excess of 0.03%, and preferably not in excess of0.003%.

The invention is also applicable to silicon relay steels which oftencontain silicon in the range from 1 to 4 /2 with adequately low carboncontents as specified above.

The process of the invention can also be used to overcome the effect ofchromium depletion at the surface, for example in straight chromium andchromium nickel steels of the stainless type, including the 200, 300,400, and 500 series of stainless steel. Thus, annealing and other heattreating operations can be carried out without special precautionsagainst chromium depletion and those stainless steels which have beenannealed in non-protective atmospheres can then be treated in theprocess described herein so as to bring about appropriate chromiumrestoration at the surface of the stainless steel.

There is evidence from immersion testing in corrosive media such as 5%aerated salt water solution at room temperature that for most favorablecorrosion resistance the carbon content in the chromized case should bebelow 0.05%. Thus where a specimen with a carbon content in thechromized case of 0.32% failed after 43 hours, specimens with carboncontents in the chromized case of 0.061%, 0.094% and 0.051% withstood120, 144 and 168 hours salt immersion respectively. Yet the chromiumcontent at the surface of the failed specimen was 30% and the chromiumcontents at the surfaces of the good specimens were 33%, 28% and 33%respectively. Other data indicate that reasonably good salt sprayresistance can be obtained if the carbon content is limited to 0.15% andfor outstanding resistance to salt immersion or salt spray the carboncontent in the chromized case should be kept below 0.05%

HEATING AND COOLING In actually carrying out the process of theinvention, it may as a matter of economy or convenience be desirable toaccomplish lower temperature heating and lower temperature cooling in anatmosphere other than an atmosphere of pure hydrogen. Thus, for examplein heating up to a temperature of 1100 F. the atmosphere, if desired,may be nitrogen or nitrogen plus hydrogen in any desired proportions butgenerally less than 8% hydrogen by volume in order to avoid explosionproblems. Similarly, at the end of the cycle the work may be cooled downfrom 1100 F. to room temperature in such an atmosphere of nitrogen ornitrogen plus hydrogen, without difiiculty. The nitrogen at these lowtemperatures is not objectionable from the standpoint of reaction withthe work to cause poor aging properties.

COIL SPACING The spacing between adjoining surfaces of one lap and thenext lap of the coil should be adequate to permit proper gas flow in theopen coil. Good results have been obtained using such spacing in theorder of 0.13 to 0.22 inch. This spacing may be maintained in a mannerwhich has been previously practiced in the annealing art, by usingspacers such as twist-over wires at the edges of the coil which haveintruding loops at the edges of turns. Also, it will be evident thatspacing can be obtained by corrugated strips or the like applied betweenlaps at the top and bottom edges of the coil.

DIFFUSION In some cases there is a tendency to deposit localized areashaving superficial chromium skins which are brittle and lacking incorrosion resistance. In order to overcome this difiiculty, it is inmany cases desirable to remove the halogen gas and hold the work at orabout chromizing temperature for an additional time to permitredififusion of this skin. A time of 1 hour at chromizing temperature indry hydrogen is sufficient. The flow of hydrogen during the rediffusionperiod need merely be sufiicient to maintain a dew oint of less than +5"F. and preferably less than -40 F., or most desirably less than -60 F.The hydrogen flow should be continued in adequate quantities to preventthe danger of building up an explosive mixture with air in the retort.

PROPERTIES OF THE CHROMIZED WORK After treatment times of hours at 1650F. using the hydrogen bromide and hydrogen system, average case depthsof 0.0010 inch have been obtained with an average chromium content of atleast 20% and as high as 23%, and a surface chromium content of at least31% and as high as 34%.

One of the important properties is that the case is quite ductile, sothat the sheet is capable of undergoing an Olsen cupping test of 0.40inch without failure of the core. The case started to orange peel at avalue of 0.25 inch. The steel is free from aging and has an aging in dexof between 0 and 2%.

Thus, the steel of the invention has a number of exceptional propertieswhich cooperate to provide good formability. An ASTM grain size of thecore of the chromized steel not coarser than 3 is readily obtained. Acarbon content in the core can readily be obtained which does not exceed0.0015% and a nitrogen content in the core can readily be obtained whichdoes not exceed 0.0005 As already explained, the steel has an agingindex between 0 and 2%. The steel can readily be obtained with an Olsencupping value of the core of at least 0.40, and an Olsen cupping valueof the case at which orange peel begins to appear of at least 0.25 inch.

One important aspect of the invention is that the carbon and nitrogencontents of the case will be related to the gage of the steel which hasbeen chromized. Starting with a decarburized plain carbon steel whichhas a carbon content not in excess of 0.002%, and chromizing on bothsides, the maximum carbon content in the chromized case will be asfollows in relation to sheet thickness:

Table VI Maximum percent carbon in case on sheet chromlzcd Thickness(in.): on both sides Typical physical propeities for the chromized plaincarbon steel with a carbon content of less than 0.0015% is tensilestrength 37,000 p.s.i.; yield strength 17,100 p.s.i.; elongation in 2inches The ASTM grain size ranges from 3 to 4.

Using titanium stabilized steel (0.05% carbon, 0.25% titanium) andchromizing at 1650 F. for 15 hours at heat, it was found thatexceptionally good mechanical properties accompanied with time grainsize were obtained. The chromizing was of normal high quality. Theterminal grain size was ASTM No. 68, while the The terminal grain sizewas ASTM No. 6-8, while the yield strength was 21,000 p.s.i. and theultimate tensile strength was 46,000 p.s.i. and the elongation in 2inches was 38%.

Table VII shows the results of chromizing according to the invention,but in several cases deliberately changing variables to illustrate theeffect of various factors. In some cases where proper control has notbeen obtained, poorer chromizing has resulted.

In all cases the steel was a 20 gage coil of the weight and carboncontent shown, the steel being plain carbon.

The weight of low carbon ferrochrome is indicated in the variousexamples. The size of the ferro-chrome particles was in every case belowinch and larger than inch.

The chromizing temperature in every case was 1650 F. to 1660 F. and thechromizing times are given.

Table VII Heat Number 841 842 864 870 954 896 897 898 902 1004 1023 10800011 Weight (lbs) 3, 000 2, 700 2, 900 3, 000 2; 200 3, 100 3, 100 3,100 3, 100 1, 800 3, 600 2, 940 Percent carbon content of steel prior tochromizing 0. 002 0. 002 0. 001 0. 001 0. 001 0. 002 0. 002 0. 002 0.002001 C) 002 Spacing between laps (in.) 0. 20 O. 20 0. 20 0. 18 18 O. 180. l8 0. 18 0. 18 0. 20 0. 15 23 welght f ferrochrorne (lbs.) 3, 600 3,590 3, 542 3, 442 3, 480 3, 600 1, 800 1, 655 815 4, 380 3, 860 2, 160Sizepf ferrochrome (1n.) /r+% /iu+%4 Ratio of area of ferrochrome,

area of steel 2. 1 2. 3 2. 0 2. 0 2. 7 2. 0 1. 0 0.92 0. 45 4. 5 3. 6 3.9 Exit dew point of hydrogen just prior to use of hydrogen I halide F.-95 -72 -64 +3 30 62 80 -65 59 42 -20 41 Percent by volume of hydrogen hlide u ed 1, 45 3, 64 7. 05 7. 82 3. 68 4. 01 3. 95 3. 75 3. 16 11. 4 9.4. 4 HBr HBr HBr HBr H01 HBr HBr HBr HBr HBr HBr HBr Chrornizing time(hrs.) 8. 5 9.5 10. 0 5. 0 10- 0 10.0 7. 0 10.0 12.0 15 15 Case depth(mils) 0. 10 1. 10 1. 10 0.90 0.50 1. 0 0.80 0.77 0.75 1. 05 1. 13 1. 01Average percent chromium in case 12 20, o 22, 9 22. 5 12 20. 5 19. 3 18.2 16. 6 22 23 Percent chromium at the surface of the case 12 26 31 12-1828 28 23 18 32 29 2 Aging index (percent) 15 1. 5 0 0-2. 8 2. 0-16. 5 00 0 0 0 0 *Coil dccarburized in same heat, but as initial lowtemperature phase prior to chromizing phase.

During chromizing in each instance a hydrogen flow of 300 to 1000 cubicfeet per hour measured at 70 F. and one atmosphere pressure was providedso as to exclude infiltration of oxygen and moisture, and permitmeasurement of exit dew point as indicated.

In Heat 841, with a concentration of hydrogen bromide in hydrogen of1.45% by volume as measured at 70 F. and one atmosphere, poor chromizingwas obtained, there being too little hydrogen bromide present.

When the concentration of hydrogen bromide as measured above wasincreased to 3.65% by volume as in Heat 842, good chromizing wasproduced. It will be noted that the steel \has an aging index of 1.5%,whereas the aging index was 15% in Heat 841, this being the normal agingindex for this steel unchromized.

In Heat 864 where the hydrogen bromide concentration as measured abovewas 7.05% by volume, it is shown that there is very slight improvementin the chromizing, although the aging index has been reduced to 0%. Inthis case there was a deposite of a great excess of chromous bromide.The porous ferrochrorne used in this heat showed a chromous bromidecontent of 245 grams per 100 pounds. This indicates a substantial levelof priming.

It is interesting to note that in Heat 842 where the 'hydrogenbromideconcentration was 3.64% by volume,

the porous ferro chrome was found to contain no chrom ous bromide.

In Heat 870, the results obtained are very similar to those of Heats 842and 864, despite the fact that the exit gases now have a high dew pointof +3 F at the beginning of the chromizing cycle, and chromizrng wasaccomplished in the relatively short time of five hours, although theconcentration of hydrogen bromide was increased to 7.82% by volume, asmeasured above.

In order to compare the benefit obtained by chromous bromide with thechromizing where chloride was used at this low temperature of 1650 F.,Heat 954 employs a concentration of hydrogen chloride of 3.68% by volumemeasured at 70 F. and one atmosphere, which is substantially comparablewith the concentration of hydrogen bromide in Heat 842. However, in Heat954 poor chrornizing was obtained, as indicated by the low case depth of0.54 mil, the low average chromium concentration in the case of lessthan 12%, the low surface concentration of chromium in the case of 12 to18%, and the wide variation in aging index between the outside of thecoil and the inside (2% to 16.5%). This is a clear indication of poorchromizing because the vapor pressure of chromous chloride is too low atthe temperature of 1650 F. to provide an effective chromizing in theparticular equipment at the circulation rate obtained. Notwithstandingthe poor chro rnizing, there was actually priming of the ferrochrome bychromous chloride to the extent of 216 grams per pounds of ferrochrome.

Heats 896, 897, 898 and 902 employ conditions which are the same exceptthat the ratio of the area of the source of chromium to the area of thesteel progressively decreases from a maximum of 2.0 in the case of Heat896 to a minimum of 0.45 in the case of Heat 902. Even in the case ofHeat 896 the average chromium content in the case and the percentage ofchromium at the surface of the case are not high enough to indicateoptimum chromizing, although they were adequate from the standpoint ofproducing a low aging index in all cases, and the product issatisfactory for many applications. The important point to note,however, is that the average chromium content in the case decreases asthe ratio of the area of the source of chromium to the area of the steeldecreases, and the same is true of the percentage chromium at thesurface of the case.

In Heat 1023 the 3600 pound AISI 1008 coil was decarburized at 1350 F.using inlet hydrogen at +l00 F. dew point, and in the same cycle, but ata higher temperature after dry out with 1l0 F. dew point hydrogen it waschrominzed using more finely crushed ferrochromiurn in the /s, sizerange.

In Heat 1080 fine enough ferrochromium 434") was used so that the weightof the 20 gauge coil (2940 lbs.) now exceeds the weight of ferrochromium(2160 lbs.).

APPARATUS FOR CARRYING OUT THE PROCESS While anyone of a wide variety ofapparatus can be used to carry out the process, we illustrate in thedrawing a very simple and convenient form which in itself is not part ofthe present invention. This apparatus was used in the above examples.

The apparatus may be in some way similar to that described by I. Arnoldin Iron and Steel Engineer, August 1960, pages 91 to 111, whichdiscusses open coil annealing, patent application Serial No. 155,585,filed November 29, 1961, now Patent No. 3,183,888, for Apparatus forSurface Coating of Strip Metal or the like.

Referring now to FIGURES 1, 2 and 3, the chromizing apparatusillustrated includes a base structure 20 in which is supported a changesupport and a dilfuser 21 having a bottom wall 22, a top wall 23, and aplurality of radially extending diffusing vanes 24. A centrifugal typefan or blower 2.5 is supported in the central portion of diffuser 21 ona shaft 26 and is adapted to be driven from a suitable power source (notshown) through a drive pulley 27 or the like. The top wall 23 ofdiffuser 21 is provided with a central opening 28 forming the inlet tothe blower 25 and the outer periphery of diffuser 21 is open at formingan annular outlet passage for the atmosphere moved outwardly by theblower 25.

Supported on the upper wall portion 23 of the diffuser 21 is a plenumchamber structure 31 having an outer wall 32, and inclined annularbaffle wall 33 and a plurality of radially extending support webs 34.Carried on these radial support webs 34 is a perforate coil supportinggrid 35, and outer imperforate ring member 36 and an imperforate centerclosure plate 37.

When it is desired to chromize a coil of strip steel, the coil is firstrewound into open form with the laps of the coil spaced apart preferablya distance equal to from one to ten times the thickness of the stripbeing treated. This may conveniently be effected by the proceduredescribed in the application of Lee Wilson and Edwin A. Corns, SerialNo. 639,939, now Patent No. 3,114,539 filed February 13, 1957. Such anopen coil is indicated at 38 in FIGURE 1 and is carried by the grid ofthe plenum chamber 31 after being placed thereon by an electromagnet orother suitable lifting means. As seen in FIGURE 1, the coil 38 isdisposed with its axis vertical and with the outer lap just overlyingthe imperferate outer ring 36 of the plenum chamber 31 and with itsinner lap just overlying the outer edge of the center closure plate 37.

Thus, it will be evident that the fan can suck the chrominzing gasesthrough the open coil in an axial direction without the possibility ofshort-circuiting to bring through the coil, gases which have not beenregenerated as later described.

A series of superimposed annular material trays 40 for the source ofchromium are arranged vertically spaced apart, in stacked or tieredrelation, as best seen in FIG- URE 3. These trays 40 are supported andheld in position by inclined annular baffle spaces 41 and verticallyextending circumferentially spaced apart spacer rods 42. Each tray 40has an annular inner wall 42' and outer wall 43, is open at the top, andhas a perforate bottom wall 44 which may conveniently be made ofexpanded metal or the like having sufficient strength and rigidity toserve its structural function while permitting the free flow ofatmosphere therethrough. On top of the perforate bottom wall 44 is alayer of wire screening 45 of sufficiently fine mesh to prevent thepassage of the source of chromium, indicated at 46 in the drawings,therethrough. Another annular ring or layer of screening 47 is placed ontop of the source of chromium on the trays 41) to prevent it beingpicked up by the atmosphere moving through the trays and carried intothe coil 38, particularly when the bed is fluidized.

As seen in FIGURE 1, the stack of trays 40 is supported on the upperwall portion 23 of the charge support and diffuser 21 just outside ofthe plenum chamber 31 and extends up in spaced relation to the outerperiphery of the coil 38. To facilitate handling, it is preferably thatthe top tray 40 of the assembly be disposed slightly below the top ofthe coil 38 so that these trays will not have to be removed each time anew coil 38 is positioned for processing.

After the open coil 38 is placed on the grid 35 of plenum chamber 31within the bank of trays 40 as seen in FIGURE 1, a removable housing inthe form of an inner cover 48 having an open bottom bell shape is placedover the trays 40 and coil 38. This housing or inner cover 48 issupported at its lower edge on the base 20 and has an outwardlyextending portion 50 having dependent flanges 51 and 52 which fit into asealing channel 53 in the base structure 20. This channel 53 containsoil, sand or other suitable sealing material to form a gas-tight sealbetween the base 20 and the inner cover 48. If desired, a water cooledgasket can be used.

To provide the necessary heat to accomplish the chromizing action abell-type furnace structure 54 is removably supported on the basestructure 20 and carries a series of circumferentially arranged heatingelements such as the radiant combustion tubes 55 of well known type. Theheat from the combustion tubes 55 is transferred through and by the wallof the inner cover 48 to the atmosphere circulating within the innercover. It will be understood that although combustion tubes areillustrated as a source of heat, electrical heating elements or othersuitable source of heat may be employed. It will also be understood thatthe base 20, diffuser 21, plenum chamber 31, trays 40, inner cover 48and furnace 54 are preferably circular in horizontal cross-sectionalform and that the furnace structure 54 is placed over the inner cover 48preparatory to a chromizing operation and is removed after the heatingoperation is completed.

In order to direct the flow of atmosphere within the inner cover 48 inthe desired path and to cause substantially all of the circulatingatmosphere to pass repeatedly through the spaces between the laps of thecoil without by-pasing or channeling around the outside or through theinside of the coil, we provide a removable top baffle member 56 whichhas a downwardly depending outer flange portion 57 adapted to rest in anannular sealing trough or channel 58 disposed at the outer periphery ofthe uppermost tray 40. Suitable granular sealing material may be carriedin the trough 58 to form a substantially gas-tight seal between the toptray 40 and the baffle member 56. This batfle member 56 is removablysupported in the channel 58 and it will be understood that it is placedin position after the coil 38 is positioned on the top of the plenum 31and before the inner cover 48 is dropped over the assembly. Other meansmay be provided for preventing the by-passing of atmosphere from theblower 25 past the trays 40, without getting through them, into the coil38. For example, a bafile wall member could be carried by the innercover 48 and extend across between the wall of the cover and the outerperiphery of the top tray 40.

As it is necessary to purge the atmosphere within the inner cover 48 ofthe air at the start of a chromizing operation, an atmosphere outletpipe 60 extends up through the base 20 up into the space within theinner cover 48 and it is provided with suitable valves and pressureregulating devices (not shown) to control the flow therethrough. One ormore gas inlet pipes 61 also extend through the base 20 and, as seen inFIGURE 1, preferably have their upper outlet ends 62 disposed closelyadjacent the source of chromium.

In the operation of the apparatus described below, assuming that thetrays 40 are properly filled with a suitable source of chromium, theopen coil 38 is placed in position on the grid 35 of the plenum chamber31, the top baffle member 56 is put in place and the inner cover 48 isthen lowered over the entire assembly. Next the belltype heating furnace54 is positioned over the inner cover 48 and the atmosphere within theinner cover 48 is purged of air by causing a suitable carrier gas, forexample a mixture of approximately nitrogen and 5% hydrogen to becharged into the inner cover 48 through the pipe 61 while permitting theescape of atmosphere through the outlet 60.

Before the purging operation, the blower 25 is started and then thefurnace 54 is fired up to proper temperature. Because of the arrangementof the baffles and support members, the flow of the atmospherecirculated by the blower 25 is, as seen from the arrows of FIGURE 1,radially outwardly from the blower 25 past the diffuser vanes 24, outthrough the outlet passage 30 and upwardly along the wall of the innercover 48 where it is heated from the cover 48. Due to the inclinedbaflle spacers 41 between the trays 40, and to the top baffle member 56,all of the atmosphere must necessarily pass upwardly through theperforated bottom 44 of the trays 40 and through the source of chromium46 thereon. The screening 47 on top of the coating material 46 preventsthe particles of chromium from being blown out of the trays. It thenpasses up along the outer surface of the open coil 38 and, as it cannotpass down through the center opening in coil 38 because of the closureplate 37, moves downwardly through the spaces between the laps of thecoil 38 in intimate contact with all parts of the entire surface of thestrip material making up the coil. After the atmosphere leaves thebottom of open coil 38, it passes through the supporting grid 35 and isdirected by the inclined bafile wall 33 to the central opening 28 of theblower 25. This circulation is continued as long as the blower 25operates.

By use of the apparatus described, the circulating atmos phere may beutilized first to heat the charge to the desired temperature withoutcoating action and then, after admission of the activating agent, forconveying the metallic chromium from the granular material 46 to, anddistributing it uniformly and evenly over, the entire surface of thestrip metal making up the open coil 38 The construction material of theretort in many cases may be plain carbon steel or low alloy steel whichwill undergo chromizing in the furnaces. There may be advantages,however, in certain cases in employing heat resisting alloys such ashigh chromium and high chromium nickel alloys including stainless steeland Inconel (79.5% nickel; 13% chromium; 6.5% iron; 0.25% manganese;0.25% silicon; 0.2% copper).

Because of the importance of avoiding oxygen and moisture within theretort, it is very desirable that any fire brick or other refractoryused in construction of the retort be sheathed or covered with metal sothat it will not be exposed to the gases within the retort.

MOTION OF THE SOURCE OF CHROMIUM If desired, the bed formed of thesource of chromium may be in motion or fluidized, by desirably blowingthe gases of the retort through the bed at a suitable velocity andemploying a particle size which is of fine enough and suflicient beddepth to provide levitation.

It will be understood that, of course, grain size control can be used sothat some of the particles of the source of ferrochrome are of largersize which will not levitate and others are of smaller size which willlevitate in the fluidized bed. Screen will suitably prevent theparticles from becoming entrained in the gases.

In view of our invention and disclosure, variations and modifications tomeet individual whim or particular need will doubtless become evident toothers skilled in the art to obtain all or part of the benefits of ourinvention without copying the process or product shown, and we,therefore, claim all such insofar as they fall within the reasonablespirit and scope of our claims.

In view of our invention and disclosure, what we claim as new and desireto secure by Letters Patent is:

1. A process of chromizing ferrous metal work, which comprises providinga retort which is closed with respect to the atmosphere, placing in theretort ferrous metal work to be chromized and closing the retort,introducing into the retort a halogen acid gas, introducing into theretort a carrier gas free from nitrogen contamination, maintaining theretort and the work at a temperature of between 1500 and 2300 F.,providing a source of chromium out of contact with the work, distributedin a series of layers through which the gas can flow sequentially, therebeing a ratio of the total surface area of the source of chromium to thetotal surface area of the work which is between 0.8 to 1 and 5 to 1,maintaining the source of chromium at a temperature between 1500 and2300 F., providing a channel from the source of chromium to the work andproviding a return channel from the work to the source of chromium,circulating the halogen gas and the carrier gas in admixture underpositive pressure sequentially through the layers of the source ofchromium and then in contact with the work to chromize the work and thenback through the source of chromium to regenerate the gas, maintainingthe chromizing potential high by withdrawing from the retort incrementsof carrier gas and 18 halogen gas and maintaining a sufficiently low dewpoint in the inlet gases, so that the dew point in the exit gas is notin excess of +5 F., and continuing the circulation of the gas and thechromizing for a time of at least 5 hours.

2. A process of claim 1, in which the carrier gas is oxygen-freehydrogen having a dew point when introduced into the retort of less thanabout 100 F.

3. A process of claim 1, in which the halogen gas is hydrogen bromideand in which the carrier gas comprises oxygen-free and nitrogen-freehydrogen.

4. A process of claim 3, in which the source of chromiurn is porous,which comprises maintaining a percentage of hydrogen bromide by volumeat 70 F. and one atmosphere pressure in relation to the chromizingtemperature which conforms with the following:

Temperature F.) Minimum Maximum 5. A process of claim 1, in which thework is stainless steel selected from the series 200, 300, 400 and 500,which has a depleted chromium content at the surface, the chromiumcontent at the surface being replenished by the chromizing.

6. A process of claim 1, which comprises holding the source of chromiumcontaining iron at a temperature higher than the temperature of the workprior to chromizing so as to diffuse chromium to the surface and diifuseiron to the interior of the source of chromium and thus replenishchromium at the surface.

7. A process of chromizing a coil of ferrous metal sheet having openspaces between laps, which comprises placing said coil in a chromizingretort with open spaces between the laps of the coil, maintaining in theretort a series of sequential layers of a source of chromium out ofcontact with the coil, establishing in the retort a circulating path forgas, which path brings the bulk of the gas within the retort intocontact with the surface of the coil between the laps and then passessuch gas sequentially through the layers of source of chromium, theratio of the total surface area of the source of chromium to the totalsurface area of the sheet being between 0.8 to '1 and 5 to 1,introducing into the retort a halogen-acid gas, introducing into theretort a carrier gas free from nitrogen contamination, continuouslycirculating said halogen gas and said carrier gas through said pathbetween the laps of the open coil and through the layers of source ofchromium, maintaining the retort, the source of chromium, and theferrous metal sheet at a temperature of between 1600 and 1800 F.,withdrawing increments of gas from the retort, the dew point of theinlet gas and the rate of withdrawal being maintained so that the dewpoint of the exit gas is not in excess of +5 F., and continuing thechromizing operation for a time of at least 5 hours.

8. A process of claim 7, in which said halogen is bromine in the form ofa material of the class consisting of hydrogen bromide, bromine andchromous bromide, and in which said carrier gas comprises hydrogen freefrom oxygen and free from nitrogen.

9. A process of claim 8, which comprises maintaining in the carrier gasa concentration of hydrogen bromide by volume measured at F. and oneatmosphere pressure shown by the following table, a chemically equiv- 19alent concentration being employed where bromine or chromous bromide isused:

Temperature F.) Minimum Maximum 10. A process of claim 9, in which theratio of surface areas is between 3 to 1 and 5 to 1.

11. A process of claim 7, in which the spacing between laps is in therange between 0.135 and 0.22.

12. A process of chromizing, which comprises placing an open coil offerrous metal sheet within a retort, maintaining within the retort abody of protective gas which is predominantly nitrogen, heating theretort and the open coil and gas to a temperature of 1100 F.,introducing a carrier gas of pure hydrogen and displacing the nitrogenfrom the retort, heating the retort and its contents to a temperaturebetween 1600 and 1800 F., introducing a halogen into the gas,circulating all gas in the retort through a circulating path whichincludes the open coil work and a source of chromium separate from thework, and thereby chromizing the work, purging the retort of the halogencontent by pure hydrogen and cooling the retort down to a temperature of1100 F. and then introducing a gas predominantly containing nitrogen anddisplacing the hydrogen from the retort.

13. A process of claim 12, in which the halogen-containing gas is a gasselected from the group consisting of hydrogen bromide, bromine andchromous bromide.

References Cited by the Examiner UNITED STATES PATENTS 2,562,467 7/1951Kinnear 29196.6 2,744,004 5/ 1956 Fraser.

2,755,537 7/1956 Smart 29196.6 2,801,187 7/1957 Galmiche 11848 X2,836,513 5/1958 Samuel 117107.2 X 2,856,312 10/ 1958 Nowak 148-6.32,962,391 11/1960 Samuel 117107.2 3,050,417 8/1962 Nack et a1 117107.2

FOREIGN PATENTS 658,683 1/1953 Great Britain.

OTHER REFERENCES Saenz: Chromizing of Steel, March 1956 (Henry Bruthcer,pp. 4-14), TH757, C552.

Arnold: Open Coil Process, August 1960, Iron and Steel Engr., 37, No. 8,pp. 91-111.

RICHARD D. NEVIUS, Primary Examiner.

JOSEPH B. SPENCER, Examiner.

1. A PROCESS SOF CHROMIZING FERROUS METAL WORK, WHICH COMPRISESPROVIDING A RETORT WHICH IS CLOSED WITH RESPECT TO THE ATMOSPHERE,PLACING IN THE RETORT FERROUS METAL WORK TO BE CHROMIZED AND CLOSING THERETORT, INTRODUCING INTO THE RETORT A HALOGEN ACID GAS, INTRODUCING INTOTHE RETORT A CARRIER GAS FREE FROM NITROGEN CONTAMINATION, MAINTAININGTHE RETORT AND THE WORK AT A TEMPERATURE OF BETWEEN 1500* AND 2300*F.,PROVIDING A SOURCE OF CHROMIUM OUT OF CONTACT WITH THE WORK, DISTRIBUTEDIN A SERIES OF LAYERS THROUGH WHICH THE GAS CAN FLOW SEQUENTIALLY, THEREBEING A RATIO OF THE TOTAL SURFACE AREA OF THE SOURCE OF CHROMIUM TO THETOTAL SURFACE AREA OF THE WORK WHICH IS BETWEEN 0.8 TO 1 AND 5 TO 1,MAINTAINING THE SOURCE OF CHROMIUM AT A TEMPERATURE BETWEEN 1500* AND2300*F., PROVIDING A CHANNEL FROM THE SOURCE OF CHROMIUM TO THE WORK ANDPROVIDING A RETURN SCHANNEL FROM THE WORK TO THE SOURCE OF CHROMIUM,CIRCULATING THE HALOGEN GAS AND THE CARRIER GAS IN ADMIXTURE UNDERPOSITIVE PRESSURE SEQUENTIALLY THROUGH THE LAYERS OF THE SOURCE OFCHROMIUM AND THEN IN CONTACT WITH THE WORK TO CHROMIZE THE WORK AND THENBACK THROUGH THE SOURCE OF CHROMIUM TO REGENERATE THE GAS, MAINTAININGTHE CHROMIZING POTENTIAL HIGH BY WITHDRAWING FROM THE RETORT INCREMENTSOF CARRIER GAS AND HALOGEN AS AND MAINTAINING A SUFFICIENTLY LOW DEWPOINT IN THE INLET GASES, SO THAT THE DEW POINT IN THE EXIT GAS IS NOTIN EXCESS OF +5*F., AND CONTINUING THE CIRCULATION OF THE GAS AND THECHROMIZING FOR A TIME OF AT LEAST 5 HOURS.